I juggle cats.
How evil is he?
As with the other boss monsters, it depends on board situation. Sometimes he's nasty, sometimes he's just another Orc.
Does the healer heal himself?
No, because he only heals when he first comes into play. When he comes into play, he's at full health. The Healer cannot give monsters more HPs than they originally have.
Does the healer use the heal action on every turn the healer is on the board? Again, even if this isn't the intended result, it sounds like a great house rule for an advanced game.
No, only when he comes into play. It's not a bad house rule, but it could make the game REALLY hard...or could just be irrelevant.